We have recently synthesized a series of new transition metal complexes with planar, tetradentate thioiminato ligands. Reactivity of several of the new cobalt(II) complexes with molecular oxygen in nonaqueous solutions containing Lewis bases will be investigated. Quantitative gas measurements and spectral studies will be undertaken to determine the stoichiometries of the oxygen adducts formed as well as the thermodynamic stabilities of the new adducts. The importance of ligand effects in determining adduct stability will be investigated by varying the substituents on the thioimine rings and bridging groups and looking for subsequent changes in metal ion reactivity toward molecular oxygen. Results will be used to evaluate the importance of inductive and pi-bonding effects in such systems. Attempts will also be made to isolate solid oxygen adducts from the solutions and to fully characterize them using a variety of physical methods.